Ubiquitous in telecommunications are conductors used for transmitting signals. Many high bandwidth conductors, such as optical fiber and Category V copper cables, tend to have a minimum bend radius below which transmission efficiency is significantly diminished. In other words, if these conductors are bent at a radius smaller than the minimum, the signal transmission losses will be prohibitively high.
The minimum bend radius is a function of the conductor type, its thickness, and the frequency of the signal being transmitted. For example, referring to FIG. 4, transmission performance in an optical fiber is shown as a function of bend radius. The plot shows the transmission loss of a 1550 nm optical signal along a Corning 900 μm SMF-28 fiber for different bend radii. From this plot it can be seen that the transmission losses become exponential as the radius becomes smaller. The point at which the loss is considered a maximum is the minimum bend radius. As used herein, the term “minimum bend radius” refers to the point at which the attenuation is reduced to 0.3 dB according to Telcordia, although we often use a lower loss as a safety factor. In the example of FIG. 4, this corresponds to about 10.5 nm (for 0.3 dB loss).
Although it is preferable to minimize bending to improve transmission efficiency in these conductors, there is a countervailing desire to make them as flexible as possible for easy installation. Consequently, conductors are typically made so flexible that they are capable of bending beyond their minimum bend radius. Therefore, there is a need to prevent a conductor from bending past its minimum bend radius even though it may be physically capable of doing so.
The need to control bending in high bandwidth conductors is heightened by their application conditions. That is, in a typical application, these conductors are terminated to connectors which plug into a backplane or other panel such that the connector is generally perpendicular to the panel. The conductor extends from the rear of these connectors and usual makes a right angle turn where it is grouped with other conductors and distributed accordingly. Since the conductor makes a right angle turn, any tension on the conductor will cause the radius of this turn to tighten. Therefore, there is a need to limit the bending of the conductor at this point.
Prior art attempts at limiting this bending involve bend limiters which are secured to the rear end of the connector and control the bending of the conductor as it leaves the connector. For example, one prior art bend limiter comprises a tubular length of elastomeric or other polymeric material which has staggered openings in the tubular length. As the bend limiter is bent and two edges of the staggered openings meet, movement is restricted with respect to that opening and further bending of the limiter must be accommodated by another opening. Openings are staggered to provide flexure at any angle in the plane transverse to the connector termination axis. Further improvements to the bend limiter include the use of a bump at the middle of the openings to limit flexure and to provide additional compliance at a higher force in a single segment to more evenly distribute compressive forces along the length of the bend limiter.
More recently, an elastic variable diameter boot has been developed as disclosed in U.S. Pat. No. 5,781,681. The profile of this bend limiter embodies the ideal elasticity model and provides for a constant bending radius for a given side load. This bend limiter represents a substantial improvement over prior art configurations.
Although this bend limiter is a marked improvement over conventional designs, the issuance of Telcordia specification GR-326, Issue III now requires that the bend limiter maintain a minimum bend radius over a range of loads. Specifically, according to the specification, a minimum bend radius needs to be maintained at a side load of 0.55 pounds at 135° and at 1.45 pounds at 90°. Such a range in load forces prevents one from optimizing a particular bend limiter for a particular load. Consequently, applicants have found that higher loads tend to buckle the bend limiter at its attachment point to the connector, while lighter loads tend to produce large curvature near the dorsal portion of the strain relief.
Therefore, there is a need for a bend limiter which does not kink the conductor at its tip at light loads or at the base at heavy loads. The present invention fulfills this need among others.